1. Field of the Invention
The present invention relates to an electronic copying apparatus and, more particularly, to an electronic copying apparatus having a function of partially changing image properties of an image reproduced from an original.
2. Description of the Related Art
As is known, an electronic copying apparatus has functions of copying an original image on a paper sheet without modifications, and of enlarging or reducing an original image and copying the image. Recently, a copying apparatus which can erase part of an original image, and copy the remaining image has been developed, and is commercially available. An apparatus of this type has a mode of erasing an image of an original in a predetermined range and copying the remaining image (masking mode), or a mode of erasing an image outside the predetermined range and copying the remaining image (trimming mode) in a manner opposite to the masking mode.
In some electronic copying apparatuses which can erase part of an original image and can copy the remaining image, image properties of an original image cannot be changed in the copied image. For this reason, the following problems occur.
More specifically, when the entire original to be copied has a low contrast and locally includes a photograph, a problem occurs in the copied image of the original. For example, in an original, e.g., old newspaper, in which a white background portion has turned yellow and a contrast between characters and the white portion is decreased, if this original includes a photograph among characters, a developing system having a large .gamma. (gamma; gradient of a characteristic curve of an image density with respect to a latent image potential) value (soft tone characteristics) is used in order to obtain a high-contrast clear image free from fogging. In this developing system, an exposure amount is set to be rather high. In this case, a copied image of the photograph portion becomes dark and unclear.
In order to clearly copy a photograph and to precisely reproduce a photograph, a developing system having a small .gamma. value (rough tone characteristics; the value is ideally 1.0, and the system must have a wide dynamic range) must be used to copy an image.
Therefore, when old newspaper including a photograph portion is to be copied, it is difficult to obtain a clear image if either system is used. Such an original is not limited to old newspaper but may be a color print of a landscape, and the like.
In some apparatus, a polarity of an original to be copied cannot be partially changed. More specifically, in some apparatuses, a positive original image cannot be partially converted to a negative image, and vice versa.